The method of the present invention utilizes a reverse osmosis technique or an ultrafiltration technique to concentrate/purify the quantity of solutes in a feed liquid. The classification of a process as either reverse osmosis or ultrafiltration depends on pore size of the membrane. Reverse osmosis membranes have a nominal molecular weight cutoff of less than or equal to 1000 daltons; ultrafiltration membranes have nominal molecular weight cutoff of greater than 1000 daltons. The selection of the membrane pore size for any specific application is based on the size of the solute which must be removed. Once the membrane is selected, a feed liquid is contacted with one side of a semipermeable membrane under pressure. The solvent and the smaller solutes in the feed liquid are then forced through the pores of the semipermeable membrane while the other solutes are rejected because of their inability to be transported through these pores due to the size of these solutes.
The present invention utilizes a reverse osmosis process to concentrate/purify a solute in a nonaqueous solution containing a mixture of solutes. A nonaqueous solution is defined as a solution that does not contain water as its major solvent constituent. Reverse osmosis membranes are typically manufactured so that the membranes are permeable to aqueous solutions. A membrane that is adapted to pass H.sub.2 O can show substantially different characteristics with nonaqueous solvents. Thus, reverse osmosis is commonly used with aqueous solutions, for example, to desalinate saline water, to concentrate fruit juices, to purify waste water and to remove water from milk.
The use of nonaqueous solutions in a ultrafiltration process is a common laboratory procedure. However, ultrafiltration membranes have not been employed to purify and concentrate taxane materials.
In the present invention, reverse osmosis and ultrafiltration are used to purify/concentrate taxanes. Taxanes are a group of diterpenes found in certain types of natural vegetation such as the Western Yew, (Taxus brevifolia). Examples of taxanes are taxol, cephalomannine, baccatin III and the like. Taxanes, specifically taxol, have become the subject of extensive research in the medical world because taxol evidences anticancer activity. Many other taxanes do not evidence this anticancer activity; however, these other taxanes are of interest because they can be synthesized in the laboratory into taxol or taxol like compounds.
The most readily available and accessible means of generating taxol is to extract it and other taxanes from plant material. There is a low taxol and taxane concentration in the biomass (i.e., the plant material) making extraction of the desired material difficult. Large quantities of solvent are required to extract the desired taxanes from the biomass. The solution resulting from this extraction process is the miscelle. Typically, the miscelle contains a minute concentrate of taxanes and a large amount of solvent and solid residue.
To facilitate isolation of the taxane material or further purification/concentration, these taxane solutions have a large portion of the solvent removed by distillation. Distillation, however, has some drawbacks. It is expensive due to the energy costs associated with heating the solvent to its boiling point. Furthermore, because taxanes decompose under distillation procedures at atmospheric pressure, vacuum distillation at reduced pressure is required. But even vacuum distillation has unresolved problems. Industrial-sized, low temperature, vacuum distillation is a difficult process due to heat transfer and vacuum problems. Distillation of a miscelle containing taxanes results in decreases in the yield of useful taxanes and increases in the cost of the final product, taxol.
Although distillation has shown many drawbacks, reverse osmosis and ultrafiltration have not been employed as methods for purifying/concentrating taxanes. Taxanes in the miscelle, specifically taxol, have a molecular weight of approximately 850 daltons. Ultrafiltration membranes having over a 1000 daltons nominal molecular weight cutoff, would be expected to pass the taxane material through these membranes showing no concentration. The crudity of the miscelle which contains not only nonaqueous solvent but also a mixture of solutes made the reverse osmosis process highly unlikely to succeed at purifying the desired solute. The reverse osmosis membranes are typically best suited to at least partially purify solutions without mixed solutes.